Polymer based insulating materials for the manufacture of electric wires and cables are known. For example, in I. Ray & D. Khastgir "Low-density polyethylene (LDPE) and ethylene vinyl acetate (EVA) copolymer blends as cable insulants", Plastics, Rubber and Composites Processing and Applications 22 (1994) 37-45, a heat-resistant cable insulant demonstrating an optimum balance of properties was sought. Blends of LDPE and EVA copolymers containing 28% vinyl acetate monomer (VA) were studied and the conclusion drawn by the authors that 50/50 LDPE/EVA blends are the best compromise in terms of electrical, thermal and mechanical properties.
In U.S. Pat. No. 5,418,272 to Kawabata et al., the inventors sought to improve conventional flame-retardant resin compositions that employ organic flame retardants. By way of this reference, an abrasion-resistant flame-retardant composition which has improved heat resistance while retaining mechanical strength, flexibility, processability and flame retardancy is provided. The composition of Kawabata et al. comprises:
I. 100 parts by weight of a polymer comprising: PA1 II. 30-200 parts by weight of an inorganic flame retardant, PA1 a. a high density polyethylene homopolymer; and PA1 b. a minor amount of an ethylene vinyl acetate copolymer having a vinyl acetate content of greater than 33.5% by weight based on the total weight of the copolymer, PA1 the composition having a volume swell no greater than: PA1 a. from about 55 to about 82 parts per hundred resins (phr) of a high density polyethylene homopolymer, and PA1 b. from about 45 to about 18 phr of an ethylene vinyl acetate copolymer having a vinyl acetate content of from about 33.5 to about 70% based on the total weight of the copolymer. PA1 a. from about 55 to about 82 phr of a high density polyethylene homopolymer; and PA1 b. from about 45 to about 18 phr of an ethylene vinyl acetate copolymer having a vinyl acetate content of from about 33.5 to about 70% based on the total weight of the copolymer. PA1 a. from about 55 to about 82 phr of a high density polyethylene homopolymer; PA1 b. from about 45 to about 18 phr of an ethylene vinyl acetate copolymer having a vinyl acetate content of from about 33.5 to about 70% based on the total weight of the copolymer; and PA1 c. the following per 100 parts of (a) and (b):
A) 50-90% by weight, based on the total polymer weight, of a polyolefinic having a maximum peak temperature (T.sub.m) of higher than 125.degree. C. as measured by differential scanning calorimetry (DSC) (e.g., HDPE), and PA2 B) 10-50% by weight, based on the total polymer weight, of a polyethylenic resin or rubber having a maximum peak temperature (T.sub.m) of lower than 125.degree. C. as measured by DSC (e.g., ethylene-vinyl ester copolymers); and PA2 25% when immersed in Reference Fuel C or CM15; PA2 20% when immersed in Reference Fuel CM20/MTBE; or PA2 7% when immersed in Reference Fuel CM85, at 60.+-.2.degree. C. for 168 hours (ASTM D 471) and having an initial elongation of at least 100% (ASTM D 412). PA2 (i) from about 5 to about 80 parts of one or more flame retardant agents; PA2 (ii) from about 0.2 to about 10 parts of one or more antioxidants; PA2 (iii) from about 0.1 to about 5 parts of one or more metal deactivators; PA2 (iv) less than or equal to 10 parts of one or more crosslinking promoters; PA2 (v) less than or equal to 20 parts of one or more reinforcing agents; and PA2 (vi) less than or equal to 2 parts of one or more processing lubricants.
where the polymer component (I) contains a functional group(s) selected from: a carbonyl group or an anhydride group thereof; an epoxy group; an hydroxyl group; an amino group; an alkenyl cyclic imino ether group; and a silane group. Component A) or B) containing a functional group participate in coupling the polymer component (I) with the inorganic flame retardant.
In U.S. Pat. No. 5,378,539 to Mark C. Chen, the inventor also sought to improve conventional flame-retardant resin compositions that employ halogens in their base resins as flame retardants. What is provided by way of this reference is a flame-retardant composition which is partially crosslinked before melt forming and which employs inorganic fire retardants which retains good mechanical properties and flame retardancy. The flame-retardant composition comprises: a) 50-95 parts of an ethylene copolymer of ethylene and at least one other monomer (e.g.,ethylene vinyl acetate copolymers); b) 5-45 parts of polyethylene provided that when the polyethylene is high density polyethylene (HDPE) or medium density polyethylene (MDPE) it does not exceed 40 wt. % of the combined weight of a) and b); and c) various named additives. This reference teaches away from HDPE levels of &gt;40 wt. % where the elongation of the resulting coating reportedly drops off significantly.
In U.S. Pat. No. 5,256,482 to Yamanouchi et al., the inventors sought to develop a crosslinked polyethylene insulated cable that showed greatly improved resistance to degradation under continuously applied voltage at high temperature for extended periods of time. The crosslinked polyethylene insulating composition that was developed is disclosed as comprising: a) a polyethylene homopolymer having a melt index of 0.5-10; b) &gt;10 to &lt;30% by weight (based on the total weight of a) and b)) of an ethylene vinyl acetate copolymer having a vinyl acetate content of 28 to 33% by weight and a melt index of 0.3 to 100; c) an organic peroxide; and d) optionally, an antioxidant. The amount of EVA having a vinyl acetate content falling within the above-referenced range is identified in this reference as critical.
In U.S. Pat. No. 4,451,536 to Barlow et al., the inventors sought to develop an improved conductive shielding for high voltage cables. Provided by way of this reference is a pliable semi-conductive resin composition which is resistant to heat distortion and which exhibits low electrical resistance. The composition of Barlow et al. comprises: 1) an ethylene-vinyl acetate copolymer containing 7 to 45% vinyl acetate monomer; 2) from about 10 to 45 wt. % of an admixture of linear low density polyethylene and HDPE; 3) a conductive component; and 4) other additives. Barlow et al. teach away from EVA copolymers having &gt;45 wt. % vinyl acetate where such copolymers may be too difficult to compound.
In more challenging environments, the performance requirements placed upon wires or cables are heightened and are often difficult, if not impossible, to completely satisfy. For example, in automotive gas-tank passthrough applications, molded wires and cables that pass through the gas tank and connect with the engine are usually subjected to high temperatures (e.g., 125-145.degree. C.) and aggressive fuels. Long term exposure to high temperatures cause a reduction in the physical and mechanical properties of the insulation material that shields and protects the wires and cables while aggressive fuels have the potential to chemically erode, swell or otherwise degrade these materials. The automotive industry has therefore dictated that wires or cables intended for use in this environment meet certain threshold requirements. These requirements include a low three-dimensional volume swell in a wide range of fuels, an initial tensile strength of at least 10 megapascals (MPa), and an initial % elongation of at least 100.
The above-referenced prior art polymer based insulating materials have proved to be unsuitable for use in automotive gas-tank passthrough applications. In the case where LDPE/EVA blends were used as the polymer matrix, the resulting insulating materials showed high to very high swelling in fuels. Insulating materials prepared from HDPE/low-VA-content EVA copolymer blends showed low elongation and/or high swelling in fuels.
It is therefore an object of the present invention to provide an insulated wire or able for use in automotive gas-tank passthrough applications capable of satisfying the rigid performance requirements set by the automotive industry.
It is a more particular object to provide a crosslinked coating composition that demonstrates excellent resistance to fuel and high temperature in addition to good mechanical properties including high elongation and tensile strength.
It is an even more particular object to provide a crosslinked coating composition that also demonstrates excellent flame resistance, high flexibility, abrasion and pinch resistance.
It is a further object to provide a melt-processable, crosslinkable blend of seemingly incompatible materials for use as an insulating coating for wires and cables to be subjected to challenging or aggressive environments.